It is generally known that iron loss of electrical steel sheets drastically increases as the excitation frequency becomes higher. Actually, however, the driving frequency of a transformer or reactor is steadily increased to achieve miniaturization of the iron core and/or improvement in efficiency. As such, the problem of heat generation due to increased iron loss of the electrical steel sheets became more apparent.
To reduce the iron loss of electrical steel sheets, it is known as an effective method to increase the Si content and thereby enhance the specific resistance. However, if the Si content in the steel sheets exceeds 3.5 mass %, workability deteriorates significantly, making it difficult to manufacture steel sheets with high Si content, by a rolling process which had been applied to manufacture conventional electrical steel sheets.
In view of the above, various methods were developed to obtain steel sheets with high Si content. For instance, JP 5049745 B discloses a method wherein an atmospheric gas containing SiCl4 is blown onto the steel sheets at high temperature of 1023° C. to 1200° C. to obtain an electrical steel sheet with high Si content. Further, JP 6057853 B discloses a method of carrying out hot rolling in manufacturing a high Si steel sheet with poor workability due to Si content of 4.5 mass % to 7 mass %.
It is effective to decrease the sheet thickness to reduce the iron loss. Among the above-mentioned methods, there is limitation in decreasing the sheet thickness by the method involving hot rolling. Thus, a method utilizing SiCl4 has been industrialized, which is referred to as “siliconizing treatment”.
However, it has been revealed that if the siliconizing treatment is applied to steel sheets with a thickness reduced to increase the Si content in the steel, the magnetic property may deteriorate. It has also been revealed that if the steel sheets are stacked on each other, as is the case in many instances, the stacking factor may significantly deteriorate.
It could therefore be helpful to provide an ultra-thin electrical steel sheet that exhibits excellent iron loss properties whereby the magnetic property is free from deterioration and degradation of the stacking factor can be avoided, even when the steel sheet with a thickness of 0.10 mm (100 μm) or less has been subjected to siliconizing treatment for increasing the Si content in the steel.